Method and system for a control scheme on power and common-mode voltage reduction for a transmitter

ABSTRACT

Provided is a method and system for controlling current characteristics in a transceiver having a transmitter. The transmitter includes a plurality of current cells. Each cell is configurable for operating in different modes. The method includes determining a first probability associated with transmitting data at a particular symbolic level and determining a second probability associated with each cell being used during a transmission at the particular symbolic level. Next, one of the modes for each cell is selected in accordance with anticipated performance requirements. An average current of the transmitter is then calculated based upon the determined first and second probabilities and the selected modes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power saving techniques for high speedtransmitters.

2. Related Art

In conventional gigabit (e.g. Ethernet) transmitters, current-modedigital to analog converter (DAC) architecture is implemented togetherwith power saving features. These power saving features allow thetransmitter to efficiently operate in different modes, namely class-A,class-AB, and class-B modes. In a current-mode transmitter, thedifferential output current defines the transmitted symbols. Desirably,common-mode current does not contribute to the definition of thetransmitted symbols. Also, in these conventional transmitters, thecommon-mode current is desirably kept as low as possible to minimize thepower consumption. Ideally, the common-mode current should be zero.

In a conventional analog front end (AFE), which typically includes aconventional transmitter, inputs of the receiver are connected through ahybrid (echo canceller) circuit to the transmitter's outputs. Thisconnection typically occurs across a duplex communications channel, suchas those used in gigabit units. The hybrid circuit cancels out the echosignals traveling back through the output of the transmitter to thereceiver inputs. This hybrid circuit, however, is only effective incanceling the differential signal. It does not cancel any undesirablecommon-mode signals.

The transmitter DAC (TXDAC) that operates in a class-AB or a class-Bmode substantially varies its output common-mode voltage as its idleoutput cells are operated in lower standby (common-mode) current to savepower. In class-A mode, the TXDAC is operated at a constant common-modecurrent that contributes no common-mode voltage variation, but consumesmore power. In conventional TXDACs, none of the currently availableclass-AB, class-B, nor class-A modes, are considered to be efficientfrom a power savings perspective.

What is needed, therefore, is a method and system that provides anefficient common-mode voltage suppression scheme that will facilitatemore efficient class-AB, class-B, and class-A operation in TXDACs. It isdesirable that such common-mode voltage suppression techniques,implemented within the TXDAC, will alleviate the need for the AFE'sreceiver to reject common-mode voltages.

BRIEF SUMMARY OF THE INVENTION

Consistent with the principles of the present invention, as embodied andbroadly described herein, the present invention includes a method forcontrolling current characteristics in a transceiver having atransmitter. The transmitter includes a plurality of current cells, eachbeing configurable for operating in different modes. The method includesdetermining a first probability associated with transmitting data at aparticular symbolic level and determining a second probabilityassociated with each cell being used during a transmission at theparticular symbolic level. Next, one of the modes for each cell isselected in accordance with anticipated performance requirements. Anaverage current of the transmitter is then calculated based upon thedetermined first and second probabilities and the selected modes.

In the present invention, an efficient programmable control scheme isprovided to achieve reasonable power savings and to reduce variations inthe common-mode voltage. Cell groups within the transmitter areselectively configured to provide the most efficient control scheme.

Further features and advantages of the present invention as well as thestructure and operation of various embodiments of the present invention,as described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings which are incorporated herein and constitutepart of the specification, illustrate embodiments of the presentinvention and, together with the general description given above and thedetailed description of the embodiments given below, serve to explainthe principles of the invention. In the drawings:

FIG. 1 is a simplified block diagram of an AFE including a gigabittransmitter and a receiver constructed in accordance with an embodimentof the present invention;

FIG. 2 is a graph of a probability distribution of outputs producedwithin the AFE illustrated in FIG. 1;

FIG. 3 is a tabular illustration of scaled down voltage levelsrepresentative of the probability distributions illustrated in FIG. 2;

FIG. 4 is a simplified functional block diagram of a TXDAC andcorresponding transmit symbol levels in accordance with an embodiment ofthe present invention;

FIG. 5 is a graphical illustration of specific probabilities associatedwith the TXDAC illustrated in FIG. 4;

FIG. 6 is a tabular illustration of output current components associatedwith transmit symbol levels and their probabilities;

FIG. 7 is a graphical illustration of actual output current valuesassociated with various modes of the TXDAC illustrated in FIG. 4;

FIG. 8 includes plots of output voltage values from a simulation of theTXDAC illustrated in FIG. 4 in accordance with an embodiment of thepresent invention;

FIG. 9 is a flowchart of an exemplary method of practicing an embodimentof the present invention; and

FIG. 10 is a block diagram illustration of an exemplary computer systemon which the present invention can be practiced.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the following detailed description is notmeant to limit the invention. Rather, the scope of the invention isdefined by the appended claims.

It would be apparent to one skilled in the art that the presentinvention, as described below, may be implemented in many differentembodiments of hardware, software, firmware, and/or the entitiesillustrated in the drawings. Any actual software code with thespecialized controlled hardware to implement the present invention isnot limiting of the present invention. Thus, the operation and behaviorof the present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

FIG. 1 is a simplified block diagram of an AFE system (transceiver) 100constructed in accordance with an embodiment of the present invention.The AFE system 100 is coupled to a transformer 102. The AFE 100 includesa main transmitter DAC (e.g. TXDAC) 104 connected to replica DACs 106and to a receiver 108. The TXDAC 104 converts received digital wordsinto analog signals and transmits the analog signals through thetransformer 102 to an external component (not shown). Since thetransformer 102 is not ideal, leakage inductance occurs.

The leakage inductance of the transformer 102 becomes a central factorin creating variations in the common-mode current to common-mode voltageand adversely effects the operation of the receiver.

The present invention provides a power savings control technique thatoperates based upon the probability of transmitted symbols. Thistechnique optimizes the power savings within the transmitter 104, whileminimizing variations in the common-mode voltage to the receiver 108.

The AFE 100 also includes a hybrid network 110, which is used to cancelout any differential signals returning back into the transmission pathfrom the transformer 102. The hybrid network 110, however, cannot cancelcommon-mode voltage input signals, which are produced from a combinationof effects of the transformer 102 and artifacts from the TXDAC 104.

As known in the art, a five level pulse amplitude modulation (PAM-5)scheme is used in gigabit Ethernet transmissions. During an Ethernettransmission, each of the PAM-5 symbols input to the TXDAC 104 isrepresented by −2, −1, 0, 1, and 2, and has an equal probability ofbeing transmitted. Before transmission, these PAM-5 symbols are filteredby a partial response finite impulse response (FIR) filter inside themain DAC.

In the exemplary embodiment of FIG. 1, the FIR filter has a transferfunction of 0.75+0.25 z⁻¹. This particular transfer function wasselected for purposes of illustration only and in no way limits orrestricts the present invention to this value. The transfer function0.75+0.25 z⁻¹ generates 17 distinct symbol output levels, out of 25possible combinations, that can be output from the transmitter 104. Theprobability distribution of these 17 output levels is depicted in FIG.2.

FIG. 2 is a graphical illustration 200 of outputs 202 that canpotentially be produced as symbol levels from the TXDAC 104. Theprobability of any one of the 17 outputs levels 202 occurring isillustrated along an axis 204. Since the output of the transmitter 104is a differential signal, each output level 202 has inverting andnon-inverting components, which are symmetrical about the zero voltagevalue, along a voltage axis 206.

In order to match the transmit level of a one volt peak differentially,the output levels 202 are scaled by one-half. The scaled down voltagelevels are −1 volt, −0.875 volts, −0.750 volts, −0.625 volts, −0.500volts, −0.375 volts, −0.250 volts, −0.125 volts, 0 volts, 0.125 volts,0.250 volts, 0.375 volts, 0.500 volts, 0.625 volts, 0.750 volts, 0.875volts, and 1 volt. FIG. 3 is a tabular illustration of this principle.

More specifically, FIG. 3 provides a tabular illustration 300 of actualtransmitted voltage values associated with the output levels 202, alsoknown as symbolic levels. In the table 300, symbolic levels 304 areassociated with actual transmitted voltages 302.

In the table 300 of FIG. 3, polarity is unimportant because the sameoutput cells are used to transmit the absolute amplitude. In essences,polarity is normalized by steering the current to either a positive ornegative terminal. Hence, both the positive and negative outputs havethe same magnitude and can be represented by one symbolic level in theanalysis that follows below.

FIG. 4 is an illustration 400 of a functional diagram of the main TXDAC104. The illustration 400 includes individual probabilities 402 of thedifferent symbolic levels 202 shown in FIG. 3. In FIG. 4, the TXDAC 104is comprised of 8 current cell groups 404-411. Each of the current cellgroups 404 through 411 further subdivides into 5 current cells, asindicated in FIG. 4. The 5 current cells are asserted by 5 differentclock signals Φ1-Φ5 that are separated from each other by about 1nanosecond (ns), for example. The 5 subgroup current cells areindividually asserted within the same cell group in order to control therise and fall times of the transmitted signal.

The graph 402 also includes 8 columns C04 through C11, which arerepresentative of current mirror group probabilities. That is, each ofthe columns C04 through C11, of the graph 402, shows the state of thecorresponding cell group 404 through 411, where different symboliclevels are being transmitted.

For example, the column C07 illustrates that the cell group 407 isactive while the symbolic levels 4 through 8 are being transmitted. Onthe other hand, the cell group 407 is idle while symbolic levels 0through 3 are being transmitted. The graph 402 also illustrates that theprobability of the current cell group 407, as indicated in C07, being inan active state is 14/25. When reading the chart 402 horizontally, itconveys information regarding which of the cell groups 404 through 411are involved in transmitting a particular symbolic level.

For example, to transmit a symbolic level 4, the cell groups 404 through407 are active, while cell groups 408 through 411 are idle.Additionally, the chart 402 conveys that the probability of transmittingthe symbolic level 4 is 4/25. The symbol transmit levels are indicatedalong a vertical axis 414 on the left side of the chart 402 and thelevel of active probabilities are illustrated in a column 416 along theright side of the chart 402.

The chart 402 of FIG. 4 also reveals that some of the current cellgroups 404 through 411 have a higher active probability than others. Theactive probabilities of current cell groups 404 through 411 are plottedin FIG. 5.

FIG. 5 is a graphical illustration 500 conveying the probability of eachof the current cell groups 404-411 being active during symboltransmission. For example, FIG. 5 illustrates that the current cellgroups 404 and 405 are active most of the time. Therefore, thecontribution of the current cell groups 404 and 405 to save power isrelatively insignificant as they are most likely to be active during atransmission. Since the current cell groups 404 and 405 are notswitching between active and idle states frequently, variations in thecommon-mode voltage are unlikely.

The graph 500 conveys that the current cell groups 406-408 spend abouthalf of their time toggling between active states and idle states. Thus,the current cell groups 406-408 disrupt the common-mode voltagefrequently, since they're switching between the active state and theidle state. This process of “active switching” changes the common-modecurrent when the TXDAC 104 is operated in either class-AB or class-Bmodes.

The graph 500 depicts that the current cell groups 409-411, however, arein an idle state most of the time. Thus, the contribution of the currentcell groups 409-411 to power savings is potentially enormous. In otherwords, power savings can be realized by reducing the large amount ofstand-by current consumption. The impact to the effects of common-modevoltage is limited, however, because of the infrequent switching.

A closed form equation can thus be derived from the chart 500 of FIG. 5,as will be discussed in greater detail below. The closed form equationcan then be used to calculate an average current consumption of theTXDAC 104 based upon the probability of individual current cell groupsbeing active.

The output current of the TXDAC 104 is composed of two components. Theoutput current of the TXDAC 104 includes the current of the idle cells(common-mode current) and current from the active cells (differentialcurrent). Data associated with these two current components is tabulatedin the illustration of FIG. 6.

FIG. 6 is a tabular illustration 600 of current components andprobabilities, associated with the TXDAC 104 of FIG. 1. In FIG. 6, afirst column 602 includes the particular symbolic level, followed by acolumn 604 representative of the idle current, and a column 606representative of the differential current. Next, the total current totransmit a particular level is computed and shown in a fourth column608. A fifth column 610 is a probability that the particular symboliclevel of column 602 will be transmitted.

S_(k) is a current scaling factor of a corresponding current cell groupC_(k). I_(x) is the current of the TXDAC 104 when it is operated inclass-A mode. S_(k) assumes values between 0 and 1. When S_(k)=0, itrepresents the corresponding current cell group being operated inclass-B mode. When S_(k)=1, it represents the corresponding current cellgroup being operated in the class-A mode. Any values between 0 and 1correspond to class-AB mode.

The sum of the products of the column 608 (total current to transmit aparticular level) and the column 610 (probability of that particularlevel occurring) produces the average current consumption of the TXDAC104 over time.

The table 600 FIG. 6, in closed equation form, is represented as:$I_{av} = {{\frac{I_{x}}{8}{\sum\limits_{k = 1}^{k = 8}{S_{k}{\sum\limits_{n = 0}^{n = {k - 1}}P_{n}}}}} + {\frac{I_{x}}{8}{\sum\limits_{n = 1}^{n = 8}{nP}_{n}}}}$

-   -   where S_(k)=1/q_(k)    -   where q_(k)=current saving factor (values from 1 to infinity)    -   where S_(k)=1, when in class-A mode    -   where S_(k)=0, when in class-B mode    -   where I_(x) is the transmitter current in class-A mode;    -   where S_(k) is the current scaling factor; and    -   where P_(n) is the probability of level n being in active state

Based upon the above equation, the average current of each of the cellgroups 404-411 can be calculated as each of the individual current cellsis placed in a different mode. Moreover, an efficient programmablecontrol scheme can be implemented to achieve reasonable power savingsand to reduce variations in the common-mode voltage. This efficientprogrammable control scheme is implemented by selectively configuringeach of the cell groups 404-411 into different modes (e.g., class-A,class-AB, or class-B) or assigning different current scaling factorsS_(k).

To better convey the effects of the current reduction technique of thepresent invention, a graph of the TXDAC 104 having its current cellgroups configured in class-A and class-AB mode, is provided in FIG. 7.

More specifically, FIG. 7 is a graphical illustration 700 of each of thecurrent cell groups (404-411) being configured in accordance with theclosed equation above. In the form of current curves 702, FIG. 7 depictsthe total average current consumption as a function of the (q) value andmode setting. Further, and for purposes of illustration, the currentcell groups 404-411 are configured in various combinations of class-Aand class-AB mode.

The various combination of class-A and class-AB were chosen for purposesof illustration only. It is to be understood that numerous othercombinations and settings are possible. Additionally, in the exemplaryillustration of FIG. 7, it is assumed that q_(k) is equal to q. In otherwords, it is assumed that each of the current cell groups 404-411 havethe same q (current saving factor) value.

In theory (not shown), the minimum current consumption is obtained bysetting all of the current cell groups 404-411 to operate in class-Bmode. With the current cell groups 404-411 in class-B mode, the bestpower savings that can be achieved is approximately 46% of the currentconsumption relative to the class-A mode. However, excessive distortionin the pure class-B mode operation makes it unsuitable in realapplication.

In FIG. 7 and at an initial value of q=1, all of the current cells404-411 were operated in class-A mode. Also at the value of q=1, I_(x)assumes an initial value of 40 milli-amps (mA). Starting with theinitial values of q and I_(x), several useful data points can beextracted from the graph 700 of FIG. 7.

For example, based on the trend of the curves 702, any further increaseof the value q beyond 10 insignificantly reduces the current. Secondly,for any value of q between 1 and 2, the power savings is fairlysubstantial, as shown in FIG. 7. Next, with the information derived fromthe graph 700 of FIG. 7, a simulation, using known techniques, can beperformed of the TXDAC 104 configured in the different modes notedabove.

Using the information derived from the graph 700 of FIG. 7, thevariations of the common-mode voltage associated with the different modesettings of the different cell groups 404-411 can be derived. Forexample, the current cell groups 406-408 switch between active and idlestates frequently. The corresponding current consumption, when thecurrent cells 406-408 that corresponds to C3-C5 in FIG. 8 are set toclass-A and class-B modes, can be calculated, as shown in FIG. 8.

FIG. 8 is a depiction of oscilloscope screen shots 800 comparingsimulation results from the current cell groups 406-408 being configuredin different current modes. The difference is 5 mA when q=5. Thecommon-mode voltage variation can be obtained from the simulationresults 800 of FIG. 8. With the current cells 406-408 set to class-A,the absolute peak variation is 72.5 milli-volts lower than when set toclass-AB mode, as shown in a top curve 802 in FIG. 8.

Based upon the curves 702 of FIG. 7 and the screen shots of FIG. 8, asystem user can individually program the current cell groups 404-411.For example, the user can apriorily tailor the current cell groupsettings to accommodate the current demands of a particular operationalenvironment.

Although the present invention is illustrated based upon the use ofeight current cell groups, any appropriate number of current cell groupscan be used in practice. Thus, the present invention is not limited tothe use of eight current cell groups.

Hence, the amount of current consumed can be controlled and operationalmodes of individual current cell groups can be selectively set back toclass-A in case excessive common-mode voltage variations begin affectingnormal operation of the receiver 108.

FIG. 9 is a flowchart of an exemplary method 900 of practicing anembodiment of the present invention. In FIG. 9, a first probabilityassociated with transmitting data at a particular symbolic level, isdetermined in a step 902. In a step 904, a second probability associatedwith each cell being used during a transmission at the particularsymbolic level, is determined.

Next, one of the modes for each cell is selected in accordance withanticipated performance requirements, as indicated in a step 906. And ina step 908, an average current of the transmitter based upon thedetermined first and second probabilities and the selected modes, isdetermined. In step 908, the determined average current reduces acommon-mode voltage back-transmitted to a receiver within the associatedtransceiver. Finally, the determined average current is implemented instep 910.

The following description of a general purpose computer system isprovided for completeness. The present invention can be implemented inhardware, or as a combination of software and hardware. Consequently,the invention may be implemented in the environment of a computer systemor other processing system.

An example of such a computer system 1000 is shown in FIG. 10. In thepresent invention, all of the elements depicted in FIGS. 5-6, forexample, can execute on one or more distinct computer systems 1000, toimplement the various methods of the present invention. The computersystem 1000 includes one or more processors, such as a processor 1004.The processor 1004 can be a special purpose or a general purpose digitalsignal processor.

The processor 1004 is connected to a communication infrastructure 1006(for example, a bus or network). Various software implementations aredescribed in terms of this exemplary computer system. After reading thisdescription, it will become apparent to a person skilled in the relevantart how to implement the invention using other computer systems and/orcomputer architectures.

The computer system 1000 also includes a main memory 1008, preferablyrandom access memory (RAM), and may also include a secondary memory1010. The secondary memory 1010 may include, for example, a hard diskdrive 1012 and/or a removable storage drive 1014, representing a floppydisk drive, a magnetic tape drive, an optical disk drive, etc.

The removable storage drive 1014 reads from and/or writes to a removablestorage unit 1018 in a well known manner. The removable storage unit1018, represents a floppy disk, magnetic tape, optical disk, etc. whichis read by and written to by the removable storage drive 1014. As willbe appreciated, the removable storage unit 1018 includes a computerusable storage medium having stored therein computer software and/ordata.

In alternative implementations, the secondary memory 1010 may includeother similar means for allowing computer programs or other instructionsto be loaded into the computer system 1000. Such means may include, forexample, a removable storage unit 1022 and an interface 1020.

Examples of such means may include a program cartridge and cartridgeinterface (such as that found in video game devices), a removable memorychip (such as an EPROM, or PROM) and associated socket, and otherremovable storage units 1022 and interfaces 1020 which allow softwareand data to be transferred from the removable storage unit 1022 to thecomputer system 1000.

The computer system 1000 may also include a communications interface1024. The communications interface 1024 allows software and data to betransferred between the computer system 1000 and external devices.Examples of the communications interface 1024 may include a modem, anetwork interface (such as an Ethernet card), a communications port, aPCMCIA slot and card, etc.

Software and data transferred via the communications interface 1024 arein the form of signals 1028 which may be electronic, electromagnetic,optical or other signals capable of being received by the communicationsinterface 1024. These signals 1028 are provided to the communicationsinterface 1024 via a communications path 1026. The communications path1026 carries signals 1028 and may be implemented using wire or cable,fiber optics, a phone line, a cellular phone link, an RF link and othercommunications channels.

In this document, the terms computer program medium and computerreadable medium are used to generally refer to media such as theremovable storage drive 1014, a hard disk installed in hard disk drive1012, and the signals 1028. These computer program products are meansfor providing software to the computer system 1000.

Computer programs (also called computer control logic) are stored in themain memory 1008 and/or the secondary memory 1010. Computer programs mayalso be received via the communications interface 1024. Such computerprograms, when executed, enable the computer system 1000 to implementthe present invention as discussed herein. In particular, the computerprograms, when executed, enable the processor 1004 to implement theprocesses of the present invention. Accordingly, such computer programsrepresent controllers of the computer system 1000.

By way of example, in the embodiments of the invention, theprocesses/methods performed by signal processing blocks of encodersand/or decoders can be performed by computer control logic. Where theinvention is implemented using software, the software may be stored in acomputer program product and loaded into the computer system 1000 usingthe removable storage drive 1014, the hard drive 1012 or thecommunications interface 1024.

CONCLUSION

A general analytic solution to calculate the current consumption of atransmitter has been introduced. The probability of the individualcurrent cell groups being used is determined. This probabilityinformation can be used to convey vital information regarding the impactof common-mode voltage variations. A user can use this probabilityinformation to apriorily program the current cell groups to meetparticular operational environment demands and/or user requirements.

Thus, using the present invention, a flexible control scheme can bedeployed to achieve a reasonable current savings while minimizingcommon-mode voltage variations. The present invention is also notlimited to a gigabit transmitter. The present invention can also beapplied to different transmission schemes and different coding methods,such as 100 TX and 10 BT Ethernet.

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

Any such alternate boundaries are thus within the scope and spirit ofthe claimed invention. One skilled in the art will recognize that thesefunctional building blocks can be implemented by analog and/or digitalcircuits, discrete components, application-specific integrated circuits,firmware, processor executing appropriate software, and the like, or anycombination thereof. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art (including the contents of thereferences cited herein), readily modify and/or adapt for variousapplications such specific embodiments, without undue experimentation,without departing from the general concept of the present invention.Therefore, such adaptations and modifications are intended to be withinthe meaning and range of equivalents of the disclosed embodiments, basedon the teaching and guidance presented herein. It is to be understoodthat the phraseology or terminology herein is for the purpose ofdescription and not of limitation, such that the terminology orphraseology of the present specification is to be interpreted by theskilled artisan in light of the teachings and guidance presented herein,in combination with the knowledge of one of ordinary skill in the art.

1. A method for controlling current characteristics in a transceiverhaving a transmitter that includes a plurality of current cells, eachcurrent cell being configurable for operating in different modes, themethod comprising: determining a first probability associated withtransmitting data at a particular symbolic level; determining a secondprobability associated with each cell being used during a transmissionat the particular symbolic level; and selecting one of the modes foreach cell in accordance with anticipated performance metrics and thedetermined probabilities.
 2. The method of claim 1, wherein theperformance metrics include reducing current consumption and minimizingcommon-mode variations.
 3. The method of claim 1, wherein the differentmodes include class-A, class-B, and class-AB.
 4. The method of claim 1,wherein the determining the first probability is a function of signalprocessing.
 5. The method of claim 4, wherein the signal processingincludes finite impulse response (FIR) filtering.
 6. The method of claim1, wherein being used during a transmission session includes being in anactive state.
 7. The method of claim 1, further comprising determiningan average current of the transmitter based upon the determined firstand second probabilities and the selected modes.
 8. The method of claim7, wherein the determining of an average current of the transmitter is afunction of a current scaling factor, a current saving factor, thedetermined probabilities, the selected modes, and a current valueassociated with the transmitter operating in a particular one of themodes.
 9. The method of claim 8, wherein the average current iscalculated in accordance with the expression:$I_{av} = {{\frac{I_{x}}{8}{\sum\limits_{k = 1}^{k = 8}{S_{k}{\sum\limits_{n = 0}^{n = {k - 1}}P_{n}}}}} + {\frac{I_{x}}{8}{\sum\limits_{n = 1}^{n = 8}{nP}_{n}}}}$where S_(k)=1/q_(k) where q_(k)=current saving factor (values from 1 toinfinity) where S_(k)=1, when in class-A mode wherein S_(k)=0, when inclass-B mode where I_(x) is the transmitter current in class-A mode;where S_(k) is the current scaling factor; and where P_(n) is theprobability of level n being in active state.
 10. A method forcontrolling current characteristics in an electrical circuit including aplurality of current cells, each being configurable for operating indifferent modes, the method comprising: determining a probability of atleast one of (i) whether each cell will be used during a transmissionsession and (ii) whether a particular symbolic level is used during theactive state; and determining an average current of the electricalcircuit based upon each of the cells being configured for one of themodes and the determined probability.
 11. An apparatus for controllingcurrent characteristics in an electrical circuit including a pluralityof current cells, each being configurable for operating in differentmodes, the apparatus comprising: means for determining a firstprobability of whether each cell will be used during a transmissionsession; means for determining a second probability of transmitting at aparticular symbolic level during the active state; means for selectingone of the modes for each cell in accordance with user preferences; andmeans for determining an average current of the electrical circuit basedupon the determined first and second probabilities and the selectedmodes.
 12. A method for controlling current characteristics in atransceiver including a plurality of current cells, each beingconfigurable for operating in different modes, the method comprising:determining a first probability associated with transmitting data at aparticular symbolic level; determining a second probability associatedwith each cell being used during a transmission at the particularsymbolic level; selecting one of the modes for each cell in accordancewith anticipated performance requirements; and determining an averagecurrent of the transmitter based upon the determined first and secondprobabilities and the selected modes; wherein the average currentreduces a common-mode voltage back-transmitted to a receiver within thetransceiver.
 13. A current-mode digital to analog converter (DAC)circuit, comprising: a plurality of current cells, each (i) beingconfigurable for operating in different modes and (ii) configured tooptimize standby/quiescent current and (iii) configured to minimize thedisturbance of common-mode voltage; and means for selecting one of themodes for each cell in accordance with user preferences.
 14. Thecurrent-mode DAC according to claim 13, further comprising means fordetermining an average current of the electrical circuit based upon thedetermined first and second probabilities and the selected modes.
 15. Amethod for controlling current characteristics in a transceiver having atransmitter that includes a plurality of current cells, each currentcell being configurable for operating in different modes, the methodcomprising: determining an average time associated with each of thecurrent cells being active; and selecting one of the modes for each cellin accordance with the determined average times.